This proposal continues a multimethod longitudinal appraisal of infant temperament and potential correlates, such as family environment and parent personality, using a sample of twins studied from birth through 30 months of age. Infant twins offer a powerful resource for detecting genetically based similarities and differences for both level and stability of temperament. Twins also provide an opportunity to detect specific contributions of family environment to both level and stability of temperament. The principal goals of the current proposal are: 1) Finish recruiting newborn twin pairs who will be repeatedly assessed through 30 months of age using the instruments described below; 2) Finish assessment of twin pairs who are currently being assessed; 3) Rescore previously obtained videotapes of twin performance during the laboratory procedures used to assess temperament; and 4) Provide further evidence on the validity of a nurses rating survey of neonatal temperament. The ultimate goal of the project is to analyze the data collected for the full sample of 280 twin pairs, using multitrait, multimethod genetic analysis. Questions to be answered by this analysis include the origins, degree and sources of intraage variability and interage stability of temperament, as well as the interrelation of genetic and environmental factors in the development of temperament. Temperament measures include assessment of neonatal irritability, reactivity and developmental status; direct observation of temperament related behaviors during standardized testing in a structured laboratory situation for seven assessments from 3 through 30 months; parent ratings on standard temperament questionnaires including the Carey Scales and Rothbart Infant Behavior Questionnaire, as well as Infant Behavior ratings obtained during mental testing. Environmental measures include a newly developed CHAOS scale, the FES scales, as well as direct observations at 7 and 30 months using codes derived from current environmental assessment techniques. Parent personality is assessed using several parent report measures.